MORPHOLOGICAL DIFFERENTIATION IN ANIMALS.
§ 242. The general considerations which preluded our inquiry into the
shapes of plants and their parts, equally serve, so far as they go, to
prelude an inquiry into the shapes of animals and their parts. Among
animals, as among plants, the formation of aggregates greater in bulk
or higher in degree of composition, or both, is accompanied by changes
of form in the aggregates as wholes as well as by changes of form in
their parts; and the processes of morphological differentiation conform
to the same general laws in the one kingdom as in the other.
It is needless to recapitulate the several kinds of modification to be
explained, and the several factors that co-operate in working them.
In so far as these are common to plants and animals, the preceding
chapters have sufficiently familiarized them. Nor is it needful to
specify afresh the several types of symmetry and their descriptive
names; for what is true of them in the one case is true of them in the
other. There is, however, one new and all-important factor which we
shall have now to take into account; and about this a few preliminary
remarks are requisite.
§ 243. This new factor is motion--motion of the organism in relation
to surrounding objects, or of the parts of the organism in relation
to one another, or both. Though there are plants, especially of the
simpler kinds, which move, and though a few of the simpler animals do
not move; yet movements are so exceptional and unobtrusive in the one
kingdom, while they are so general and conspicuous in the other, that
the broad distinction commonly made is well warranted. What, among
plants, is an inappreciable cause of morphological differentiation,
becomes, among animals, the chief cause of morphological
differentiation.
Rooted animals or animals otherwise fixed, of course present traits
of structure nearest akin to those we have lately been studying. The
motions of parts in relation to one another and to the environment,
being governed by the mode of aggregation and mode of fixing, we are
presented with morphological differentiations similar in their general
characters to those of plants, and showing us parallel kinds of
symmetry under parallel conditions. But animals which move from place
to place are subject to an additional class of actions and reactions.
These actions and reactions affect them in various ways according to
their various modes of movement. Let us glance at the several leading
relations between shape and motion which we may expect to find.
If an organism advances through a homogeneous medium with one end
always foremost, that end, being exposed to forces unlike those to
which the other end is exposed, may be expected to become unlike it;
and supposing this to be the only constant contrast of conditions,
we may expect an equal distribution of the parts round the axis
of movement--a radial symmetry. If, in addition to this habitual
attitude of the ends, one surface of the body is always uppermost
and another always lowermost, there arise between the top and bottom
dissimilarities of conditions, while the two sides remain similarly
conditioned. Hence it is inferable that such an organism will be
divisible into similar halves by a vertical plane passing through
its axis of motion--will have a bilateral symmetry. We may presume
that this symmetry will deviate but little from double bilateralness
where the upper and under parts are not exposed to strongly-contrasted
influences; while we may rationally look for single bilateral symmetry
of a decided kind, in creatures having dorsal and ventral parts
conversant with very unlike regions of the environment: as in all cases
where the movement is over a solid surface. If the movement, though
over a solid surface, is not constant in direction, but takes place as
often on one side as on another, radial symmetry may be again looked
for; and if the motions are still more variously directed--if they are
not limited to approximately-plane surfaces, but extend to surfaces
that are distributed all around with a regular irregularity--an
approach of the radial towards the spherical symmetry is to be
anticipated. Where the habits are such that the intercourse between the
organism and its environment, does not involve an average equality of
actions and reactions on any two or more sides, there may be expected
either total irregularity or some divergence from regularity.
The like general relations between forms and incident forces are
inferable in the component parts of animals, as well as in the animals
as wholes. It is needless, however, to occupy space by descriptions of
these. Let us now pass to the facts, and see how they confirm, _à
posteriori_, the conclusions here reached _à priori_.